1. Field of the Invention
This invention relates to an automatic focus adjusting apparatus for use in a camera or the like.
2. Related Background Art
Many of the automatic focus adjusting systems of single-lens reflex cameras intend to make the lens infocus to an object by repetitively effecting the cycle of "focus detection (sensor signal input and focus detection calculation) and lens driving". The amount of lens driving in each cycle is based on the defocus amount at the point of time whereat focus detection has been effected in the cycle, and this presumes that the defocus amount during focus detection is eliminated at the end of lens driving.
As a matter of course, focus detection and lens driving require a correspondingly long time, and in the case of a stationary object, the defocus amount does not vary unless the lens is driven and therefore, the defocus amount to be eliminated at the point of time whereat lens driving has been terminated is equal to the defocus amount at the point of time whereat focus detection has been effected, and proper focus adjustment is accomplished.
However, in the case of an object which is in motion, the defocus amount varies during focus detection and lens driving and said defocus amount to be eliminated sometimes differs remarkably from the detected defocus amount and as a result, this leads to the problem that at the end of lens driving, the lens is not in focus to the object.
An automatic focus adjusting method which intends to solve the above-noted problem is disclosed in Japanese Laid-Open Patent Applications Nos. 62-125311, 62-139511 and 62-139512.
The gist of the method disclosed in these publications is to foresee a variation in the defocus attributable to the movement of an object and exert correction on the amount of lens driving (hereinafter referred to as the pursuit correction) in view of the variation in the detected defocus in each said cycle and the time intervals between the cycles, and from the viewpoint of the focusing accuracy at the end of lens driving, an improvement in the above-noted problem is expected by the same method.
However, when said pursuit correction is actually effected, the following problem arises.
When the object in the distance measuring field shifts to another object when the object is being pursued in the pursuit correction mode, the continuity of the variation in the imaging plane position is lost and therefore, if foreseeing is effected with the aid of the data of the past object and the data of the new object, wrong foreseeing will be performed and as a result, the lens will be driven to an entirely different position.
Thus, when the object in the distance measuring field shifts to another object, wrong foreseeing is performed, and this leads to the problem that the wrong foreseeing is not eliminated as long as foreseeing control is effected by the use of the data of the old object.
The operation of the above-described prior-art apparatus will hereinafter be described with reference to the accompanying drawings.
FIG. 2 is a graph for illustrating the lens driving correction method according to the prior art. In the figure, the horizontal axis represents time t, and the vertical axis represents the imaging plane position x of the object.
The solid-line curve x(t) represents the imaging plane position at time t of the object which comes near the camera in the direction of the optic axis when the photo-taking lens is at infinity. The broken line l(t) represents the position of the photo-taking lens at time t, and the infocus condition is brought about when x(t) and l(t) coincide with each other. [t.sub.i, t.sub.i' ] represents the 25 focus detecting operation, and [t.sub.i', t.sub.i+1] represents the lens driving operation. Also, in the example of the prior art shown in FIG. 2, the assumption is made that the imaging plane position varies in accordance with a quadratic function. That is, if the current and past three imaging plane positions (t.sub.1, x.sub.1), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3) are known, the imaging plane position x.sub.4 at time t.sub.4 in TL (AF time-lag+release time-lag) after time t.sub.3 can be foreseen on the basis of the equation x(t)=at.sup.2 +bt+c.
However, what can be actually detected by the camera are not the imaging plane positions x.sub.1, x.sub.2 and x.sub.3, but the defocus amounts DF.sub.1, DF.sub.2 and DF.sub.3 and the amounts of lens driving DL.sub.1 and DL.sub.2 converted into the amounts of movement of the imaging plane. The time t.sub.4 is a future value and actually, it is a value which varies with a variation in the accumulation time of an accumulation type sensor caused by the brightness of the object, but here, for simplicity, it is assumed as follows: EQU t.sub.4 -t.sub.3 =TL=TM.sub.2 +(release time-lag) (1)
Under the above assumption, the amount of lens driving DL.sub.3 calculated from the result of the focus detection at time t.sub.3 can be found as follows: EQU x(t)=at.sup.2 +bt+C (2)
If (t.sub.1, l.sub.1) in FIG. 2 is considered to be the origin, ##EQU1##
If the equations (3), (4) and (5) are substituted into the equation (2) to find a, b and c, ##EQU2##
Consequently, the amount of lens driving DL.sub.3 converted into the amount of movement of the imaging plane at time t.sub.4 can be found as follows: ##EQU3##
A problem which arises when the object in the distance measuring field shifts to another object will now be described with reference to FIG. 3.
FIG. 3 shows the relation between time and the imaging plane position, and in this figure, the solid line represents the imaging plane position of a first object, and the dot-and-dash line represents the imaging plane position of a second object.
Here, let it be assumed that at times t.sub.1 and t.sub.2, focus detection is effected for the first object and the lens is driven and at time t.sub.3, focus detection is effected for the second object.
Thereupon, on the camera side, the imaging plane positions x.sub.1, x.sub.2 and x.sub.3 ' at times t.sub.1, t.sub.2 and t.sub.3 are calculated from the defocus amount and the amount of lens driving obtained by focus detection, and a quadratic function f(t) passing through (t.sub.1, x.sub.1), (t.sub.2, x.sub.2) and (t.sub.3, x.sub.3 ') is calculated, and the imaging plane position x.sub.4" at time t.sub.4 is foreseen by means of this f(t).
However, the imaging plane position of the first object at time t.sub.4 is x.sub.4 and the imaging plane position of the second object at time t.sub.4 is x.sub.4', and x.sub.4" obtained by foreseeing is a position differing from the imaging plane positions of both of the objects.
Thus, to foresee the imaging plane position x.sub.4 of the first object, it is necessary to find a function passing through (t.sub.1, x.sub.1), (t.sub.2 x.sub.2) and (t.sub.3, x.sub.3), and to foresee the imaging plane position x.sub.4' of the second object, it is necessary to find a function passing through (t.sub.1, x.sub.1'), (t.sub.2, x.sub.2') and (t.sub.3, x.sub.3').
On the camera side, however, the first object and the second object cannot be distinguished from each other and therefore, foreseeing calculation is effected by the use of the defocus amount obtained by focus detection at time t.sub.3. As a result, the foreseeing function is neither an approximate function of the imaging plane position of the first object nor an approximate function of the second object, and the foreseen lens driving position becomes wrong. This is a problem which will arise certainly if the main object is switched to the second object while the photographer is pursuing the first object, because the wrong foreseeing as described above takes place if the data of the focus detection effected for any other object than the main object is present in the data used for foreseeing.